The present invention is particularly found to be useful when employed in police radar detectors. Generally, police radar devices are used in the detection and measuring the speed of moving vehicles, and an industry has developed whereby radar detectors are provided to the public, to be employed in vehicles of all sorts to determine the presence of a police radar in the vicinity of the vehicle. Also, many jurisdictions are beginning to utilize so-called "photo radar" devices, whereby a radar device constantly scans oncoming traffic for the presence of vehicles exceeding a posted speed limit, and thereafter a camera is operated to take a photograph of the vehicle as it leaves the immediate vicinity of the device so as to capture an image of the license plate of the vehicle on film. The radar detector industry has developed, not so much as to develop circumstances where a driver of a vehicle might drive his vehicle at an unnecessarily high speed simply because he feels he can do so with impunity due to the lack of police radar in the immediate area, but rather so as to forewarn drivers of the presence of police radar particularly in jurisdictions that use police radar and speeding tickets solely for the purposes of enriching the public coffers in that jurisdiction.
In North America, there are three bands of radar frequencies in which police radar devices presently operate: they are the X-Band, at 10.525 GHz; the K-Band, at 24.150 GHz; and the K.sub.a -Band, which is centred about 34.7 GHz.
There are, of course, several approaches to designing and developing radar detectors that might detect the presence of police radar at those somewhat disparate frequencies. It will be noted that the difference between X-Band and K-Band is more than one octave, and between X-Band and K.sub.a -Band is nearly two octaves, and yet police radar detectors are expected to operate efficiently and with increased sensitivity in all three bands. At the same time, the public expects to purchase such radar detectors at low prices, thereby forcing the manufacturers to improve their designs so as to meet the public demand for low cost, high sensitivity radar detectors that are capable of operating in three bands.
Some of the most successful commercial products that have been brought to the market, in the past, are those manufactured by the Assignee herein and which embody inventions particularly as defined and described in MARTINSON U.S. Pat. No. 4,961,074, issued Oct. 2, 1990, and U.S. Pat. No. 4,952,936 issued Aug. 28, 1990. Those two patents describe, respectively, multi-band radar detectors having a mixer in which direct current bias may be utilized so that the radar detector will monitor two different radar frequencies by application and non-application of the DC bias to the mixer, using a single local oscillator; and radar detectors that utilize a plurality of local oscillators so as to provide differing local oscillator frequencies to the mixer in order to achieve an appropriate resultant frequency or intermediate frequency. The common threads throughout those two patents are, however, that they rely upon the use of dielectric resonator local oscillators, and the requirement to use more than one local oscillator if more than two different radar frequencies are to be detected. The use of dielectric resonator first local oscillators permitted very effective coupling and decoupling of one or another of the multiple dielectric resonator local oscillators to a common microstrip feedline, so as to provide discrete local oscillator signals to the local oscillator input port of the mixer.
However, the use of multiple dielectric resonator local oscillators is costly; and it also precludes any possibility of sweeping a local oscillator through a band of frequencies since dielectric resonator local oscillators can operate only at a single, fixed frequency.
An earlier patent, also issued to Martinson, was U.S. Pat. No. 4,630,054 issued Dec. 16, 1986, and also commonly owned with the present application and the previously referred to patents. That patent disclosed a radar detection and signal processing circuit that could function at both X-Band and K-Band, where the incoming radar signal was mixed with a local oscillator output that had a specific frequency such that the resultant mixer product intermediate frequency from either X-Band or K-Band was a consequence of the fundamental or a second harmonic of the local oscillator frequency. The intermediate frequency may then be mixed with a signal from a swept frequency oscillator and then directed to a bandpass filter and other signal processing circuitry, to actuate an alarm. Because the local oscillator frequency was chosen so that its resultant frequency when mixed with X-Band or K-Band signals was a particular intermediate frequency signal, then signals from other than the X-Band or K-Band would not mix with the local oscillator output to produce the same intermediate frequency signal, and thus any other signals other than those at the required at the intermediate frequency could be effectively ignored.
The same principles effectively apply in the present application; that is, it is the purpose of the present invention to provide a mixer for a radar detector or otherwise, as discussed hereafter, where incoming signals from a variety of disparate frequency bands may be mixed with a local oscillator signal in such a manner that the resultant frequency--the intermediate frequency--is the absolute value of the incoming signal frequency plus or minus the fundamental frequency of the local oscillator, or twice that frequency (the second harmonic), or three times that frequency (the third harmonic).
Some further explanation of the operation of a mixer having a single oscillator follows:
First, a mixer element can be considered to be a two-terminal device; a single diode is a common example. The output from a local oscillator is generally a sinusoidal voltage at a fundamental frequency, at which oscillator energy is delivered to the mixer. However, the mixer--for purposes of this explanation, a single diode--conducts only when the voltage of the oscillator signal is above the threshold voltage of the diode, and only when the oscillator signal is in the correct positive-going or negative-going sense, depending on the orientation of the diode. The reflection coefficient of the diode ideally has the form of a square wave, it is either +1 or -1, with an abrupt transition from either state to the other. Therefore, for each cycle of an oscillator having a sinusoidal waveform, a single diode will conduct only once, and only during that half of the sinusoidal waveform that is in the correct sense and only when the voltage of the waveform exceeds the threshold voltage of the diode.
With no bias applied to a single diode mixing element, the effect of the diode threshold voltage is to cause the time duration of diode conduction to be less than the time duration of diode non-conduction in any given period. This causes the introduction of even harmonics in the diode reflection coefficient, and can be utilized to enhance mixer products arising from even harmonics of the diode reflection coefficient--in particular, the second harmonic in radar detector applications. This improved mixing at even harmonics comes at the expense of mixing products resulting from the fundamental and odd harmonics of the diode reflection coefficient. However, application of a suitable bias to the diode can equalize the conduction and non-conduction time intervals so as to enhance fundamental and odd harmonic mixing, while at the same time degrading even harmonic mix products.
MARTINSON U.S. Pat. No. 4,961,074 discusses an alternative arrangement where a pair of antiparallel diodes is used. In that case, the conduction waveform of the mixer diodes when the mixer is not biased has two conduction cycles for each one cycle of the oscillator signal. In other words, the conduction waveform has a frequency which is twice the fundamental frequency of the local oscillator. However, when a bias is applied, then the local oscillator output is shifted so that only one or the other of the diodes will be conductive, and the conduction waveform of the mixer has a fundamental frequency which is equal to the fundamental frequency of the local oscillator. These factors are discussed in greater detail hereafter.
The inventors herein have discovered that, quite unexpectedly, if differing values of bias--a first zero DC component, or a second level or third level DC component, where the second level is higher than the third level--are imposed across a mixing element, and the mixing element is a two-terminal device that has substantially symmetrical non-linear forward and reverse voltage/current characteristics, then conversion loss in the mixing element may be minimized so as to maximize the magnitude of the resultant frequency mixer product signal at frequencies that are the absolute values of the mixing product of an incoming signal plus or minus the frequency of the local oscillator, or its second or third harmonics.
Assuming a first input signal at a frequency of f.sub.IN, and a mixing element reflection coefficient that is essentially a square wave in nature as a result of the non-linear mixing elements being driven by a sinusoidal local oscillator, then the signal at frequency f.sub.IN is multiplied by the square wave reflection coefficient waveform, which may have frequency components f.sub.LO, 2f.sub.LO, 3f.sub.LO, 4f.sub.LO, etc., depending on the relative time duration of positive and negative portions of any one cycle. This multiplication will give rise to product terms that include frequencies .vertline.f.sub.IN .+-.f.sub.LO .vertline., .vertline.f.sub.IN .+-.2f.sub.LO .vertline., .vertline.f.sub.IN .+-.3f.sub.LO .vertline., etc., where the magnitude of each term is proportional to the magnitude of the respective term f.sub.LO, 2f.sub.LO, 3f.sub.LO, etc., in the reflection coefficient waveform.
Thus, the present invention provides a mixer for converting input signals from a first signal source at a frequency of f.sub.IN, where the mixer comprises a mixing element, a local oscillator having a fundamental frequency f.sub.LO, means for conducting the input signals to an input signal port of the mixing element, means for conducting signals from the local oscillator to a local oscillator signal input port of the mixing element, and means for conducting a maximized resultant frequency signal away from an output port of the mixing element.
In a particular aspect of the present invention, the mixer further comprises a DC bias source for imposing a DC component of bias voltage across the mixing element. The magnitude of the resultant frequency signal may thus be maximized by a selection of a DC bias so as to minimize conversion loss in the mixing element at any one of the following group of frequencies: .vertline.f.sub.IN .+-.f.sub.LO .vertline., .vertline.f.sub.IN .+-.2f.sub.LO .vertline. and .vertline.f.sub.IN .+-.3f.sub.LO .vertline.
The mixing element is a two-terminal device, and it is a condition of the present invention that the mixing element has a substantially symmetrical non-linear forward and reverse voltage/current characteristic.
The DC bias source further comprises means to adjust the level of the DC component of bias voltage which is imposed across the mixing element to any one of three levels, being a first zero DC component, or a second DC component, or a third DC component. The second DC component of bias voltage that is imposed across the mixing element is higher than the third DC component of bias voltage across the mixing element.
Moreover, the mixing element must have a pair of conduction threshold voltages which are substantially symmetrical above and below zero volts, beyond which the mixing element will be conductive at least when a signal from the local oscillator is imposed on the mixing element. The signal from the local oscillator has a substantially sinusoidal voltage waveform, and its peak-to-peak voltage is greater than the voltage difference between the pair of conduction threshold voltages of the mixing element.
In the presence of the substantially sinusoidal waveform signal from the local oscillator, when the first zero component of bias voltage is imposed across the mixing element, the voltage across the mixing element is centred about zero volts and it passes through each of the conduction threshold voltages in a respective symmetrical positive-going and negative-going sense. When the second DC component of bias voltage is imposed across the mixing element, the voltage across the mixing element is shifted by the amount of the second DC component of bias voltage with respect to zero volts in the same sense as the bias voltage is directed, so that the voltage across the mixing element is above or below zero volts for a first substantial portion of each cycle of local oscillator signal, and below or above zero volts for only a second small portion of each cycle. The magnitude of the value of the voltage across the mixing element during that first portion exceeds the threshold voltage in the same sense as the voltage shift for about 40% to about 60% of the period of that cycle, so that the mixing element is conductive in its respective direction during that 40% to about 60% of the period of the cycle. Conversely, the magnitude of the value of voltage across the mixing element during the second small portion of the cycle is less than the threshold voltage in the opposite sense as the voltage shift is applied, so that the mixing element is not conductive at any time during that second smaller portion.
However, when the third DC component of bias voltage is imposed across the mixing element, the voltage across the mixing element is shifted by the amount of the third DC component of bias voltage with respect to zero volts in the same sense as the bias voltage is directed, so as to be above or below zero volts for a first portion of each cycle, and below or above zero volts for a second portion of each cycle of local oscillator voltage. The magnitude of the value of voltage across the mixing element during the first portion exceeds the threshold voltage in the same sense as the voltage shift for about 35% to about 55% of the period of that cycle, so that the mixing element is conductive in its respective direction during that 35% to about 55% of the cycle. Moreover, the magnitude of the value of the voltage across the mixing element during the second portion of the cycle also exceeds the threshold voltage in the opposite sense as the voltage shift is directed, so that the mixing element is conductive in the opposite direction of the DC bias voltage shift for a third period of time during that second portion of the cycle.
Thus, the mixing element has an idealized reflection coefficient waveform that has a fundamental frequency which is twice the fundamental frequency of the local oscillator when the first zero DC component of bias voltage is imposed across the mixing element, so as to give the maximized resultant frequency .vertline.f.sub.IN .+-.2f.sub.LO .vertline.. Also, the idealized reflection coefficient waveform has a fundamental frequency which is equal to the fundamental frequency of the local oscillator when the second DC component of bias voltage is imposed across the mixing element, so as to give the maximized resultant frequency .vertline.f.sub.IN .+-.f.sub.LO .vertline.. Finally, the idealized reflection coefficient waveform has a large amplitude component that is at a frequency which is three times the fundamental frequency of the local oscillator, when the third DC component of bias voltage is imposed across the mixing element, so as to give the maximized resultant frequency .vertline.f.sub.IN .+-.3f.sub.LO .vertline..
The mixer element may be a planar doped barrier diode, which will exhibit substantially symmetrical non-linear forward and reverse voltage/current characteristics. Also, if the mixer is to be utilized at lower frequencies than radar frequencies per se, such as for other purposes of mixing radio frequency signals, the mixing element may be a pair of zener diodes that are connected in antiseries--that is, back-to-back.
Particularly when the mixer of the present invention is being used in radar detectors, the mixing element may be a pair of antiparallel diodes. One particularly useful embodiment is, therefore, utilization of mixer in keeping with the present invention, together with a single local oscillator, in a radar detector that is adapted to detect radar frequencies in X-Band, K-Band, and K.sub.a -Band.